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This finding from a small Swedish study suggests that there’s now hard science to support the long-held belief that sufficient rest produces “beauty sleep.”

Up to now, “the concept of beauty sleep has lacked scientific support,” the study authors write.

Researchers in Europe enrolled 23 healthy volunteers between the ages of 18 and 31 to take part in the study. Each was photographed on two occasions -- once after normal sleep of eight hours, and once after being deprived of sleep (31 hours of being awake after five hours of sleep).

The participants weren’t allowed to wear makeup and underwent similar grooming procedures, such as shaving. And each was asked to try to look relaxed and project a neutral facial expression for both photographs.

Sleepers and Nonsleepers Rated by Untrained Observers

Sixty-five untrained people rated the photographs for attractiveness, tiredness, and healthy appearance.

Overall, the raters judged the sleep-deprived faces as less healthy, less attractive, and more tired. However, a few of the study participants’ sleep-deprived photos were rated healthier, more attractive, and less tired.

The conclusion -- if you don’t get enough sleep, it’s going to show on your face.

The research shows that people “are sensitive to sleep-related facial cues,” which has potential implications for social and clinical judgments, as well as behavior.

“Generally, human judgment involves complex processes, whereby ingrained, often less consciously deliberated responses from perceptual cues are mixed with semantic calculations to affect decision-making,” the authors write. “Thus, all social interactions, including diagnosis in clinical practice, are influenced by reflexive as well as reflective processes in human cognition and communication.”

Because sleep disorders are very common, additional research is needed on signals of fatigue and ill health resulting from sleep deprivation, according to the authors.

Study author John Axelsson, PhD, of the Karolinska Institutet in Stockholm, Sweden, tells WebMD in an email that it is yet to be determined what the exact facial features are that signal to observers that people aren't getting enough sleep, but there's no doubt that facial features do reflect tiredness.

"This study shows that sleep is important for facial features and hence makes up the first scientific evidence of the phenomena of 'beauty sleep,'" he says. "This suggests that sleep is important for how we look and are treated in a wide range of social situations."